Recent surveys indicate that between 10 and 20 percent of homes in the United States have indoor radon levels that exceed EPA's guideline level for remedial action, resulting in an estimated 5,000 to 20,000 lung cancer deaths each year. This estimate is based on extrapolating from results of studies of miners with very high radon exposures; findings from these studies may not be generalizable to the population at large. Risks to women and children have not been studied. The relationship between cumulative residential exposure to radon and cancer risk is being evaluated in an ongoing collaborative study involving Yale University and the University of Utah. The study is enrolling 1000 smokers and 750 nonsmokers with lung cancer as well as 2100 population controls from Connecticut, Utah, and Southern Idaho. The study is designed to evaluate the potential interaction between radon and cigarette smoke exposure. Detailed residential and exposure histories are being obtained, and radon measurements are being made in past homes of participants using year-long alpha track etch detectors, in order to estimate cumulative radon exposure since age 25 for each subject. Complete lifetime exposure assessments (including childhood) will be made for a subset of participants. A companion study in Connecticut involving 125 children with cancer and 250 cancer-free controls is evaluating the potential childhood cancer risk associated with residential radon exposure. A third study has been developed in collaboration with a cooperative childhood cancer treatment group; radon exposure will be assessed in an ongoing study of risk factors for childhood acute nonlymphocytic leukemia.